The present invention relates to intensifying screens employed in X-ray radiography or the like, radiation receptors therewith, and radiation inspection devices therewith.
In X-ray radiography employed in medical diagnosis and non-destructive inspection for industrial purpose, in general intensifying screens are used in combination with X-ray film to enhance system sensitivity. An intensifying screen is generally formed by sequentially forming a phosphor layer and a relatively thin protective film on a support consisting of paper or plastic.
In recent years, reduction of subject""s exposure to radiation in medical diagnosis or the like is strongly demanded. In order to cope with this demand, in X-ray radiography, high-speed X-ray films or high-speed X-ray intensifying screens are used to reduce subject""s exposure. In order to enhance sensitivity of X-ray film, high speed X-ray films are generally used. In order to enhance sensitivity of intensifying screens, phosphors of high emission efficiency are employed.
When X-ray films or intensifying screens are made highly sensitive, there occur the following problems. That is, when the high-speed X-ray films are employed, though lowering of sharpness is small, granularity is deteriorated. By contrast, when the high-speed intensifying screens are employed, there also occurs deterioration of granularity. Recognizability of a subject in X-ray radiography involves both of granularity and sharpness. Deterioration of granularity deteriorates in particular the recognizability of subjects of low contrast.
From the above, with an object to improve image quality of intensifying screens, various improvements of phosphor layers have been attempted. For instance, when a phosphor layer is produced by the use of a kind of settling method named xe2x80x9cRyuen Houxe2x80x9d in Japanese, a phosphor layer of which particle size distribution becomes smaller from the protective film side toward the support side, a structure in which particle size is graded can be obtained (Japanese Patent Publications (KOKOKU) No. Sho 55-33560 and No. Hei 1-57758). This kind of structure of phosphor layer can enhance speed and sharpness of intensifying screens.
However, the aforementioned intensifying screens of structure of graded particle size distribution are produced by drying solvent while letting settle phosphor particles in phosphor slurry by the use of gravity. Accordingly, it takes long time for produce to result in pushing up the production cost. In Japanese Patent Publications (KOKOKU) No. Sho 55-33560 and No. Hei 1-57758, a structure of multi-layers of phosphors of different particle sizes is disclosed. These patent publications disclose only examples of the structure of graded particle size distribution but does not disclose detailed conditions of each phosphor layer or the like.
By contrast, Japanese Patent Laid-open Publication (KOKAI) No. Sho 58-71500 discloses an intensifying screen in which the surface side of a phosphor layer thereof is constituted of larger phosphor particles of an average particle diameter of 7 to 20 xcexcm, and interstices of the larger phosphor particles and support side thereof are constituted of phosphor particles of an average particle diameter of 4 xcexcm or less. According to such an intensifying screen, sensitivity and sharpness can be improved by some degree. However, granularity can not be sufficiently improved.
In Japanese Patent Laid-open Publication No. (KOKAI) Hei 8-313699, there is disclosed an intensifying screen having a plurality of phosphor layers the support side of which layers is composed of phosphor particles of smaller average particle diameter. Each phosphor layer of this intensifying screen, when each average particle diameter of phosphor particles constituting each phosphor layer is R and particle size distribution thereof is "sgr", satisfies a relation of 0 less than "sgr"/Rxe2x89xa60.5, respectively. Furthermore, in this patent publication, among the plurality of phosphor layers, the phosphor layer of the protective layer side has an average particle diameter of 10 to 20 xcexcm and the phosphor layer of the support side has an average particle diameter of 1 to 5 xcexcm.
Thus, in an intensifying screen having a plurality of phosphor layers, when particle size diameters of phosphor particles constituting the respective phosphor layers are stipulated similarly, sufficient improvement of sharpness and granularity is not necessarily obtained. By the experiments carried out by the inventors, it has been found that when a plurality of phosphor layers is composed of a plurality of phosphor particles of different average particle diameters, according to average particle diameters of the respective phosphor layers, various kinds of conditions have to be set.
As mentioned above, high speed intensifying screens due to the use of phosphors of high emission efficiency can be effective in reduction of subject""s exposure and in improvement of sharpness, however, cause a problem of deterioration of granularity. On the contrary, when phosphors of low emission efficiency are used, the granularity can be improved but the sharpness deteriorates. Thus, there is a certain degree of reciprocity between radiographic performance.
As to such problems, existing intensifying screens having a structure composed of single phosphor layer can not satisfy both of granularity and sharpness. The intensifying screens having a structure of graded particle diameter distribution are relatively satisfactory with respect to speed and sharpness. However, it takes longer time for formation of phosphor layer to result in pushing-up of manufacturing cost and at the same time due to fluctuation of manufacturing conditions, large performance variation is invited. Further, in the existing intensifying screens having a plurality of phosphor layers of different average particle diameters, the sharpness and granularity have not been sufficiently improved.
In contrast, radiation is used not only for radiography of medical diagnosis but also for treatment of subjects. A device for radiotherapy is one in which a high energy X-ray beam of approximately 4 MV obtained from a linear accelerator called linac is irradiated to an subject to cure. Before beginning treatment with a device for radiotherapy, in order to confirm reproducibility of a portion being exposed that is set by treatment program, radiography or TV imaging is carried out with the beam being used for treatment.
However, there is a problem that in the aforementioned high energy X-rays, when an X-ray image is taken with an ordinary intensifying screen after transmission of X-rays of a subject, sufficient contrast can not be obtained. To this end, so far, a fluorometallic screen that is composed of integration or superposition of an ordinary intensifying screen and a metallic plate such as lead alloy foil or copper plate, and medical X-ray film or industrial X-ray film are combined to employ. Silver halide in film emulsion has the maximum of spectral sensitivity at 45 kV. Accordingly, a high energy X-ray beam of 1 MV or more is absorbed less to result in poor efficiency. This is the reason why the fluorometallic screen has been employed.
A fluorometallic screen is composed of a phosphor layer of such as CaWO4 in contact with a lead alloy foil, for instance. In such a fluorometallic screen, after appropriate absorption of a high energy X-ray beam at the lead alloy foil, a sensitizing effect due to emission of phosphor, an elimination effect of scattered X-rays due to the metallic foil, a sensitizing effect of phosphor due to secondary electrons due to Compton scattering or the like can be obtained.
However, there is a problem from an environment point of view as to handling of foils of lead alloy. Other than this, plate of heavy metal such as tungsten has been taken up. However, tungsten plate is much expensive that there is a problem when being put in practice. In contrast, a fluorometallic screen employing copper plate is small in X-ray absorption, that is, insufficient in absorption of high energy X-rays of 1 MV or more. In addition, existing fluorometallic screens are insufficient in speed, sharpness or the like, and recognizability of portions being treated is poor.
An object of the present invention is to provide multi-purpose intensifying screens improved in speed, sharpness, granularity or the like.
A first more concrete object of the present invention is to provide an intensifying screen employing phosphor of high emission efficiency in which, while preventing deterioration of speed and sharpness from occurring, granularity is improved and mass-productivity is satisfied. In addition, another object of the present invention is, by employing such intensifying screens, to provide a radiation receptor and a radiation inspection device that realize reduction of for instance subject exposure and improve capability of diagnosis.
A second more concrete object of the present invention is to provide an intensifying screen that has sufficient absorption of high energy X-rays of 1 MV or more, for instance, and is improved in handling performance during manufacture and usage, and in speed and sharpness.
In order to look into likelihood of improving performance of an intensifying screen that has a plurality of phosphor layers of different average particle diameters, the present inventors have carried out detailed experiments concerning particle diameter and particle size distribution of phosphor particles constituting the respective phosphor layers, and packing density of the respective phosphor layers or the like. As the result of these experiments, it is found that the particle size distribution and packing amount of each phosphor layer are required to be controlled within an appropriate range according to the average particle diameter of phosphor particles constituting each layer.
A first intensifying screen of the present invention comprises a support, a first phosphor layer disposed on the support and constituted of particles of a first phosphor of which average particle diameter is D1 and range coefficient k, which expresses particle size distribution, is in the range of 1.3 to 1.8, a second phosphor layer disposed on the first phosphor layer and constituted of particles of a second phosphor of which average particle diameter is D2 that satisfies D2 greater than D1 and range coefficient k, which expresses particle size distribution, is in the range of 1.5 to 2.0, and a protective layer disposed on the second phosphor layer.
A second intensifying screen of the present invention comprises a support, a first phosphor layer disposed on the support and constituted of particles of a first phosphor having an average particle diameter of D1, a second phosphor layer disposed on the first phosphor layer and constituted of particles of a second phosphor having an average particle diameter of D2 that satisfies D2 greater than D1, and a protective layer disposed on the second phosphor layer, wherein when a coating weight per unit area of the particles of the first phosphor in the first phosphor layer is CW1 and a coating weight per unit area of the particles of the second phosphor in the second phosphor layer is CW2, the ratio of the CW1 and CW2 (CW1:CW2) is in the range of 8:2 to 6:4.
A radiation receptor of the present invention comprises an X-ray film, a front intensifying screen laminated along a surface of the subject side of the X-ray film and consisting of an intensifying screen of the present invention, a back intensifying screen laminated along a surface opposite to that of the subject side of the X-ray film and consisting of an intensifying screen of the present invention, and a cassette accommodating a laminate of the front intensifying screen, the X-ray film and the back intensifying screen.
A radiation inspection device of the present invention comprises a radiation source, and the aforementioned radiation receptor of the present invention that is disposed opposite to the radiation source through a subject.
Here, it is known that particle size distribution of powder such as phosphor particles can be approximated by lognormal distribution in most cases. That is, when particle diameter is d, x=log d, an average at this time is xcexc, and standard deviation is "sgr", probability density function f(x) can be given by the following formula.
f(x)=(1/"sgr"{square root over (2 xcfx80)})xc2x7(exp [xe2x88x92(xxe2x88x92xcexc)2/2"sgr"2])
A probability of x being x0 and less is called a cumulative distribution function F (x0) and is expressed by the following formula.       F    ⁡          (      x0      )        =            ∫              -        ∞            x0        ⁢                  f        ⁡                  (          x          )                    ⁢              xe2x80x83            ⁢              ⅆ        x            
Phosphor particles being measured are put in a dispersion medium such as water and are dispersed well to measure particle size distribution by the use of Coulter counter method, micro-track method or the like. An average particle diameter of a phosphor is obtained as a median value of this particle size distribution.
FIG. 9 shows an example of a cumulative particle size distribution (in terms of weight) of a phosphor employed in intensifying screens of the present invention. In the figure, points show actual measurement data and a curved line shows a theoretical cumulative distribution of lognormal distribution decided so that average value xcexc and standard deviation "sgr" thereof meet the measured values. From this example, particle size distribution of phosphor is evident to be expressed well by the lognormal distribution. The particle size corresponding to 50% of vertical axis of this cumulative distribution curve is a median value of this particle size distribution and denoted as average particle diameter D. Width of particle size distribution can be characterized by range coefficient k.
The range coefficient k is defined as follows. When summation of weight of particles in the range of D/kxe2x88x92kD (total weight) is 68.2689% of the weight of whole particles, k is defined as a range coefficient. That is, k is a number of more than 1, the larger the value of k is, the broader is the particle size distribution, and the closer to 1 the k is, the sharper is the particle size distribution.
The first and second intensifying screens of the present invention have a phosphor layer of two-layer structure. A first phosphor layer thereof is formed on support side and consisting of particles of phosphor of smaller particle diameter, and a second phosphor layer thereof is formed on protective film side and consisting of particles of phosphor of larger particle diameter. In an intensifying screen of phosphor layer of two-layer structure, by narrowing the particle size distribution of phosphor particles of smaller particle diameter and by making relatively broader the particle size distribution of phosphor particles of larger particle diameter, sharpness and granularity can be improved. Further, by setting smaller the coating weight per unit area of particles of phosphor of the first phosphor layer constituted of particles of phosphor of particle diameter smaller than that of the second phosphor layer constituted of particles of phosphor of larger particle diameter, sharpness and granularity can be improved.
In the intensifying screen of the present invention, the phosphor layer of two-layer structure can be produced by applying an ordinary producing process as identical as the case of the ordinary phosphor layer. Accordingly, in addition to manufacture of intensifying screens themselves being easy, aimed performance can be obtained with reproducibility. Radiation receptors and radiation inspection devices of the present invention, due to adoption of the aforementioned intensifying screens, in particular even when radiography system is made highly sensitive, can obtain excellent recognizability.
The third intensifying screen of the present invention intends to enhance the contrast of radiographs taken with X-rays of high energy such as for instance 1 MV or more, and to improve speed, sharpness and granularity thereof.
That is, a third intensifying screen of the present invention comprises a support, a phosphor layer disposed on the support, a protective film disposed on the phosphor layer, and a powder layer. Here, the powder layer is disposed between the support and the phosphor layer and is consisting of at least one kind of particles selected from particles of simple metal, particles of alloy consisting mainly of metal and particles of compound consisting mainly of metal. Here, a thickness of the powder layer is in the range of 2 to 40 kg/m2 in terms of weight per unit area. As metals to be used for the third intensifying screen, at least one kind of heavy metals such as W, Mo, Nb and Ta can be cited.
In the third intensifying screen of the present invention, a powder layer composed of particles of heavy metals such as W, Mo, Nb and Ta that are large in absorption of X-rays of high energy or composed of particles consisting mainly of heavy metal is disposed between a support and a phosphor layer. Such powder layer absorbs the X-rays of high energy up to an appropriate state corresponding to exposure speed of X-ray film. Accordingly, excellent contrast that can be applied to medical diagnosis can be obtained. Further, scattered X-rays can be effectively absorbed due to the powder layer and a sensitizing effect of phosphor due to secondary electrons based on Compton scattering can be obtained. As a result of these, speed, sharpness and granularity can be improved.